The present invention relates to an optical beam scanning device which is applicable to color printers with a plural-drum type, color copy machines, digital copy machines, digital multi function peripherals, and so forth and an image forming apparatus in which the optical beam scanning device is employed.
For example, in an image forming apparatus such as digital multi function peripherals with a plural-drum type, a plurality of image forming units each of which corresponds to a color component obtained by color separation are employed, and also optical beam scanning units each of which provides each of the image forming units with a laser beam, namely, image data corresponding to a color component are used. Each of the image forming units transfers an image of a color component associated with itself onto a recording medium.
Here, in order to obtain a preferable color image, it is necessary that, with respect to each color component, positions and distances in the main-scanning direction and positions and distances in the sub-scanning direction or the like are congruent with one another or have in appropriate relationships and in order to meet the needs, an adjusting mechanism or the like is provided.
For example, U.S. Pat. No. 5,774,274 discloses a technique in which an assembly of mirrors are used whose angles in the main- and sub-scanning direction are different with respect to each laser beam of respective color components so that laser beams of respective color components are guided to one horizontal synchronization sensor, and timing of the each color component is taken by means of detection outputs of laser beams having different reflection angles. It is also disclosed that a technique in which, with respect to each color component, the number of laser beams of respective color components is two or more, the position in the sub-scanning direction is detected, and on the basis of the result obtained, the actuators each of which controls the distance between the beams associated with the same color component are moved, whereby image surface beam distances are kept to a constant value.
Further, it is disclosed that a technique in which, by means of the registration sensor, written registration measuring image data is read, and on the basis of the result of the reading and so on, registration correction information is prepared.
However, a technique in which an assembly of mirrors whose angles are respectively different in the main-scanning and sub-scanning directions with respect to each of the laser beams of the color components are used to guide the laser beams of the respective color components to one horizontal synchronization sensor, a large swing angle of the polygon mirror is needed for causing the respective color component beams to be incident onto the horizontal synchronization sensor.
For this reason where the effective field angle as an entire optical system (sum of an angle provided for controlling the beam position and image effective field angle) is the same, a problem is caused that the image effective field angle (area) becomes narrow. Also, where the image effective field angle (area) is the same, a problem is caused that deflection angle, which is used to secure properties such as the relative positions between beams, image formation properties and the like become substantially large. There is a further problem that since the scanning optical system is required to secure properties including the imaging characteristic, fθ characteristic, inter-beam pitch (pitch between beams) in the main- and sub-scanning directions and so on for the horizontal synchronization sensor located outside the imaging area in the main-scanning direction, it becomes difficult to secure an imaging characteristic and so on in an entire apparatus, and the sizes of the optical elements for imaging and a polygon mirror become large.
Further, there is still another problem that since detection is and control of their positional relationships are performed with respect to each of the color components constituted by a plurality of laser beam, the constructions for the detection and control become complicated or the processing for the detection and control become complicated.
Still further, it is required that the position of the registration sensor is accurately adjusted and kept.
With respect to the main-scanning direction of the exposure onto the photosensitive drum by means of the scanning optical system, when the succeeding factors occur, with reference to the position of the horizontal synchronization sensor, magnification varies at almost the same rate in an entire area.                i) deviation of magnification in the main-scanning direction caused by the deviation of wavelengths of the laser beam by temperature change,        ii) deviation of magnification in the main-scanning direction caused by thermal expansion of the optical components, housing, positioning components in the device and so on, and        iii) deviation of magnification caused by the change in distance between the optical elements and the photosensitive drum.        
Under the technique in which the magnification in the main-scanning direction is measured on the basis of the output of the registration sensor, and the magnification thus measured is corrected by means of image clocks, such a problematic situation arises as the registration correction causes unintentional magnification deviation for the succeeding reason. Specifically, when the distance between a pair of registration sensors is deviated at an initial stage or in the course of the operation, the magnification in the main scanning direction being measured becomes an incorrect value. However, it cannot be discerned whether the magnification deviation is caused by the above-mentioned factors i) to iii) or by the distance deviation in the registration sensors.
For the above-mentioned reasons, it is required accurately to adjust the positions of the registration sensors so as to keep the position thus adjusted but when a positional displacement caused by, for example, secular change or the like occur, the above-mentioned problems occur inevitably.